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Smart locks: what fits, and what happens when it fails
Retrofit against replacement, whether your door will take one, and the question most buying guides skip entirely.
Every lock fails eventually, and a lock is the one device where failure means standing outside your own house. That question comes before brand or price.
On this page
Start here
Retrofit or full replacement
This decision comes before brand, features or price, because it determines what is even possible on your door.
| Retrofit | Full replacement | |
|---|---|---|
| What changes | The inside thumbturn only. The outside of the door is untouched. | The whole lock, inside and out. |
| Your keys | Keep working exactly as before. | Usually replaced, though some accept a key as well. |
| Renting | Generally acceptable, since nothing visible changes and it reverses. | Wants the landlord's agreement in writing. |
| Look | The existing hardware stays, so the door looks unchanged from outside. | A visible change, which some people want and some do not. |
| Entry methods | Limited. The outside is still a keyhole. | Keypad, fingerprint, card, phone. Whatever the unit offers. |
In rented accommodation, read the tenancy agreement before ordering anything. A retrofit that leaves no trace is a different proposition from replacing the lock, and the second usually needs permission on paper.
Compatibility
Whether it will fit at all
Doors vary more than product pages suggest, and a lock that does not fit is a return rather than a project. A few measurements settle it.
- Door thickness. Every lock states a range it accommodates. Older doors and doors with added insulation sometimes fall outside it.
- The existing mechanism. Whether it is a deadbolt, a multipoint system, a mortice lock or a night latch changes what can replace it entirely.
- Backset. How far in from the door's edge the centre of the hole sits. Two common sizes exist and they are not interchangeable.
- Clearance behind the door. A smart thumbturn is bulkier than a plain one, and can foul a wall, a radiator or a coat hook.
- Whether the door actually closes cleanly. A door that needs a shove to latch will defeat an automatic lock, and motorising it makes the problem louder rather than better.
That last one matters more than any specification. A lock that has to fight a misaligned frame will drain its cells quickly and fail unpredictably. Where a door is sticking, that wants sorting before anything electronic goes on it.
Entry methods
The ways of getting in, compared
| Method | Strengths | Worth knowing |
|---|---|---|
| Keypad | No phone needed. Codes can be given to a visitor and withdrawn afterwards. | Worn keys eventually reveal which digits are used. Some units randomise a prefix to counter this. |
| Phone | Nothing to remember. Some unlock as you approach. | Depends on a charged phone and a working connection. Proximity unlocking is convenient and worth thinking about carefully. |
| Fingerprint | Quick, and nothing to carry or recall. | Reading is less reliable with wet or cold hands, which is precisely when you are standing outside. |
| Physical key | Works with no power, no network and no phone. | Whether one exists at all differs by model, and it is the single most important thing to check. |
Establish what happens when the cells die before you buy. Some locks accept a key. Some take an external power pack held against a contact. Some have neither, and that is a decision to make deliberately rather than discover.
The important part
What happens when it fails
Every lock fails eventually, and a lock is the one device where failure means standing outside your own house. Most guides treat this as a footnote. It deserves more.
Four things to establish in advance
- The low-battery warning. Most give several weeks of notice. Learn what the warning looks like rather than meeting it for the first time on a wet evening.
- The backup route. A key, an external power contact, or nothing. Know which, and if it is a key, know where it is.
- What happens in a power cut. Battery locks carry on. Mains-powered ones may not, and the hub they depend on certainly will not.
- What happens if the manufacturer closes. A lock whose app stops working may still open by key or keypad, or may not. Local control matters more here than in any other category.
Fire safety, which is not optional
Any lock fitted to a final exit door has to allow escape without a key from the inside. Rules differ by country and by whether the building is rented, and this is not an area for guessing. Where there is any doubt, the local authority or a qualified locksmith is the right place to ask.
The radio
How smart locks connect
A lock runs on cells and has to last months, which rules out Wi-Fi for most designs. Wi-Fi radios are power-hungry in a way that suits a mains-powered plug and not a battery-powered lock.
- Bluetooth only. Works from a phone within range. No remote access, no hub, longest battery life. Perfectly adequate if you only ever unlock while standing there.
- Zigbee, Z-Wave or Thread. Low-power mesh, needing a hub or border router. Gives remote access while keeping battery life reasonable. The usual arrangement.
- Wi-Fi directly. Exists, and generally means either much shorter battery life or mains power.
- A manufacturer's own bridge. A small plug-in unit that gives remote access to an otherwise Bluetooth lock. Sometimes included, frequently sold separately.
Check whether a bridge is included before buying. A lock advertised with remote access sometimes needs a separate purchase to deliver it, and that is rarely prominent on the box.
Day to day
Guest codes and shared access
This is where a smart lock earns its cost, more than the unlocking itself. A code can be issued, limited and withdrawn without anybody handing over a key or getting one cut.
- Time-limited codes for a cleaner, a dog walker or a tradesperson, expiring on their own afterwards.
- Codes that work on certain days, which suits a recurring arrangement.
- A record of who came and went, which is useful and also worth thinking about, because it is a record of your household's movements.
- Withdrawing access without warning, which is the part that genuinely beats a key.
Worth considering before turning it all on
An access log is a detailed account of when a house is empty. Where that record lives, who can see it, and what happens to it if the company changes hands are reasonable questions to ask before enabling everything a lock offers.
Frequently encountered
Common situations, and what usually causes them
- Cells drain far faster than advertised. Usually a door that does not close cleanly, so the motor is straining on every cycle. Alignment first, replacements second.
- The lock turns but the door does not open. The bolt is binding against a misaligned strike plate. A common outcome of a door dropping slightly over time.
- Remote access stopped working, local still fine. The bridge or hub, not the lock. Worth confirming that unit is powered and connected before suspecting anything else.
- The keypad ignores a correct code. Some units lock out temporarily after several wrong attempts. Others struggle in extreme cold.
- Auto-lock fires while somebody is still in the doorway. The delay is usually adjustable, and the default is frequently shorter than a household wants.
- A guest code stopped working early. Check its schedule. Time-limited codes expire on the lock's own clock, which can drift if it has been offline.
Where a situation persists beyond these, the manufacturer holds the warranty and model-specific information. Anything involving the door itself, the frame or fire escape requirements belongs with a qualified locksmith or joiner.
Keep reading
Locks rarely use Wi-Fi directly
Most run on cells for months, which means a low-power radio and usually a hub. The protocols guide explains what that involves.
Read the Wi-Fi guideHow we checked this
Where this comes from
- The literature each manufacturer issues for its own equipment, which is where intended behaviour is set down
- What Apple and Google publish for people building software, since that is where handset permissions and radio handling are described
- The wireless specifications themselves, which govern how each band and every low-power protocol actually conducts itself
- Opening the applications and observing them in whatever state they are in today, with every check dated
Nothing stated here rests on a forum thread or an article with no name attached to it. Where a point simply cannot be pinned down, the page admits as much instead of papering over it.
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